Mass and energy balance in the near-surface layers of a cometary nucleus

In the near future, several space missions are scheduled, which will closely investigate short-period comets. Some of these (e.g., ROSETTA) will also deliver landing probes for in situ investigations of the nucleus' surface. Therefore there is now renewed interest in the structure and behavior of cometary surface layers. In this paper we discuss in detail some basic features that may constitute the "landscape" of a comet nucleus: gas outflow from gaps and holes is considered, based on a fundamental kinetic approach and previous work. Both the gas emission from ice-dominated walls and the outflow through "dusty" channels with a sublimating icy bottom are calculated. We attribute special emphasis to the influence of recondensation on the energy balance. It is found that-assuming realistic temperature profiles as a function of depth-there exist areas of net recondensation along the walls of ice channels, which might act as interior heat sources. Furthermore, our Monte Carlo model is applied to interpret some results obtained in various comet simulation (KOSI) experiments, which were performed in the years 1989-1993 with porous ice samples irradiated in the space simulator at DLR Cologne/Germany, It was found that when using the measured temperature profile inside the ice sample Monte Carlo calculations predict that the recondensation region coincides with the region of the steepest temperature gradient. Another result relevant for the understanding of cometary surface phenomena is that at least in first order the temperature difference between a sublimating ice front and the surface temperature of an overlying porous dust mantle or nonvolatile cohesive residuum is almost independent of the thickness of the nonvolatile layer. (C) 1999 Academic Press.